1. Field of the Invention
The present invention generally relates to wireless communication systems. More particularly, the present invention relates to a base station, a receiving device, a mobile terminal, and a frequency sharing method.
2. Description of the Related Art
There is a known method (hereafter called a first method) for sharing the same frequency band by multiple systems in short-distance communications using, for example, ultra-wideband (UWB). In the first method, transmission power of a system is limited to a low level based on a transmission power mask determined statically according to relationships with other systems sharing the same frequency band.
There is another known method (hereafter called a second method) used, for example, in wireless LANs. In the second method, a terminal determines whether an allocated frequency band is being used by a neighboring terminal before starting communications and refrains from transmitting a signal while the neighboring terminal is transmitting a signal.
There is still another known method used in a mobile communication system employing a frequency division multiplexing scheme such as FDMA. In this method, when there are multiple cells adjacent to each other, each cell is divided into an outer area and an inner area and different frequencies are allocated to the inner and outer areas (see, for example, Japanese Patent Application Publication No. 2005-80286 and “Mobile WiMAX—Part I: A Technical Overview and Performance Evaluation”, WiMAX Forums March, 2006). For example, as shown in FIG. 1, a frequency commonly used by all the cells is allocated to the inner area where interference by other cells is low and a frequency used by only a part of the cells is allocated to the outer area where interference by other cells is high.
However, the above related art technologies have problems as described below.
With the first method, because transmission power control is not performed flexibly according to operating conditions of other systems, it is difficult to improve frequency use efficiency. Also, it is theoretically difficult to apply the first method to a communication system such as a cellular system where signals are transmitted in a wide area using comparatively high transmission power.
In the second method, transmission conditions in a frequency band adjacent to that of other systems are not taken into account. Also with the second method, because whether it is possible to transmit a signal is determined based only on surrounding conditions without taking into account transmission parameters, it is difficult to efficiently use frequency resources.